Can I import an image file so it can be used in grease pencil?
HOW TO ADD BACKGROUND OR REFERENCE IMAGES IN BLENDER 2D GREASE PENCIL THIS EVEN APPLIES TO GENERAL MODE

When you start in 2D animation Mode. Your option will be set to "DRAW" mode instead of 
"object mode" you need to Switch from Draw mode to object Mode.

When you switch to object mode you will notice new button options and one of them is "Add"
you can use that or just Shift A.

1 -shift+A to add background image or reference Image.

2 - Shift + A to add Grease Pencil then select blank or stroke or the monkey if you don't know what they each mean, google it. if you just want to draw select blank.

3 - now change mode from object mode to "DRAW MODE" 
    enjoy.